1. Field of the Invention
The present invention generally relates to path protection methods and layer-2 switches, and particularly relates to a path protection method and a layer-2 switch used in a bridge network.
2. Description of the Related Art
Bridge networks were mainly used in LANs (Local Area Networks) in early days. Since then, their use has been spreading to carrier networks as signified by the fact that the bridge networks are now referred to as “wide-area Ethernet (registered trademark)”.
When bridge networks are used for carrier networks, there is a demand for improvement in fault tolerance with respect to bridge networks, apparatuses constituting the bridge networks, and links between the apparatuses constituting the bridge networks.
To this end, redundancy configuration is employed at various levels, including redundancy in the channel cards of the apparatuses, redundancy in the control cards of the apparatuses, redundancy in links between the apparatuses, etc. Also, a redundancy protocol is used to collect network topologies for the purpose of path control.
Conventionally, when there is a need to achieve redundancy between an end and an end in a bridge network, the spanning tree protocol (which was the standard in versions prior to the 2004 version) or the rapid spanning tree protocol (which is the standard from the 2004 version) is used as defined in IEEE802.1D to achieve redundancy by use of layer-2 switches.
In the following, what is referred to as “spanning tree protocol” means the rapid spanning tree protocol. Since the functions of the spanning tree protocol are included in the rapid spanning tree protocol, a description will not be given with respect to the functions of these individual protocols separately.
A related-art bridge network will be described with reference to FIG. 1 and FIG. 2.
Switches #1 through #6 are layer-2 switches, which constitute a bridge network. The switch #1 and the switch #6 are connected to a user terminal or to a user network that is not controlled from the bridge-network side. The switch #1 and the switch #6 are called end nodes because they are located at the ends of the bridge network. The switches #2 through #5 have no connection to an external terminal or network. Since the switches #2 through #5 relay traffics passing through the bridge network, these switches are referred to as relay nodes.
The spanning tree protocol may be introduced in this bridge network to perform path control. In this case, as shown in FIG. 1, blocking ports BP#1, BP#2, and BP#3 are created at the locations where a physical loop is formed by the ports between the switches. Provision is then made such that frames other than those used in the spanning tree protocol cannot pass through these blocking ports. In this manner, a network topology that has no logical loop is constructed.
As shown in FIG. 2, a failure may occur at the link between the switch #2 and the switch #3. In such a case, the physical loop disappears, resulting in the blocking port #3 being changed into a forwarding port, thereby making a path change.
Patent Document 1 discloses the use of the VLAN ID (Virtual LAN Identifier) in the VLAN tag of an IEEE802.1Q frame as a logical identifier, thereby defining a P-P (point-to-point) connection between an edge and an edge as a P-P logical path.
[Patent Document 1] Japanese Patent Application Publication No. 2003-258829.
When a redundancy protocol such as the spanning tree protocol is used in a bridge network, the following problems may arise.
First, there is a need to make the same redundancy protocol operate at all the layer-2 switches constituting the bridge network. Because of this, it is extremely difficult to install the protocol in a currently operating network. Second, layer-2 switches that support a redundancy protocol are generally provided at higher per unit price, thereby requiring higher equipment investment.
Third, the redundancy protocol is controlled by software. While the software responsible for such control is being upgraded, switching may be performed at another layer-2 switch. This may affect the main signals. Forth, even if the VLAN tag standardized according to IEEE802.1Q is used, traffics may be concentrated on a single path since the network topology is not constructed on a per-VLAN-tag basis.
Fifth, a time length on the order of seconds may be required from the detection of failure in the network to the recovery from the failure (it may even be on the order of tens of seconds according to the original spanning tree protocol). Sixth, no traffic can go through a port serving as a blocking port. Because of this, the blocking ports cannot be used for the purpose of distributing the load.
Accordingly, there is a need for a path protection method and a layer-2 switch that can provide path protection by making low equipment investments that only expand the function of end nodes.